1. Field of the Invention
The present invention relates to a method for treating organic sludges, particularly for treating organic sludges, such as, sewage sludge, etc., including the steps of solubilizing the sludge, and then digesting the solubilized sludge in an anaerobic condition.
2. Related Art Statement
Recently, treatment and disposal of organic sludges produced in large amounts in sewage-treating plants has become a serious social problem. Anaerobic digestion of an organic sludge improves treatability of the sludge to improve stability, innoxiousness, volume reduction, and dehydration property, etc., of the sludge when used as a reclaiming substance, and recovery of methane gas as a valuable resource from the sludge. The recovering of methane gas as a valuable resource has been adopted in sewage-treating plants and the like as a useful method for treating organic sludges because of increased amounts of recovered methane due to qualitational change of organic components of organic sludges resulting from the consumers' life of the current society and because of development of a technique for generating electric power from the recovered methane gas, and many sewage-treating plants have been operated using one phase reaction type reaction tank at a medium digestion temperature of about 37.degree. C. However, at present, such method are still low in digestion ratio and insufficient in volume reduction and methane gas recovery and reduction of organic matter concentration. Therefore, in order to solve such problems many methods have been researched and studied of heating a sludge (heating method) to accelerate solubilization of the sludge at a prestige of the anaerobic digestion and thereafter treating the solubilized sludge in an anaerobic condition. These methods are summarized as follows.
In these methods, an organic sludge is agitated under heating at around 60.degree. C. in a heating tank, and then anaerobically treated in an anaerobic digestion tank. If the methods are applied to an organic sludge like a sewage sludge resulting from microorganisms, a low solubility value of 10-30% is merely obtained which is not worth the thermal energy applied for heating, so that a low digestion ratio of 50% at the most can merely be obtained, and hence predominance of the anaerobic treatment of organic sludges is not yet settled. Moreover, the digestion gases obtained by the above conventional methods have low methane contents, so that they could not increase electric power generation efficiency of electric fuels or the like when supplied thereto as a raw material gas for electric power generation.
Furthermore, in separating solids from liquid phase of a digested sludge, conventionally a gravitational concentration method has been mainly used. However, such a gravitational concentration method can not sufficiently separate solids from liquid phase of a digested sludge, so that they have problems as follows.
.circle. 1 Concentration of the concentrated digested sludge is so low that the abilities for the subsequent dehydration process and drying process are deteriorated. PA0 .circle. 2 Solids and the like are existing in intermingled state in the liquid phase of the digested eluate, so that the load of a subsequent water treating system is increased if the digested eluate is directly returned as a return water to the water treating system. PA0 .circle. 3 In the anaerobitic digestion process, phosphorus of ortho phosphorus state is dissolved out from the solubilized and digested sludge, so that the concentration of phosphorus of ortho phosphorus state in the digested eluate is high.
Another problem of the gravitational concentration method is that the separation speed of the method separating the solids from the liquid phase of digested sludge is so low that the capacity of the solid/liquid separation tank must be equally large to that of the anaerobic digestion tank.
Meanwhile, a solid/liquid separation method which obviates the problems of the gravitational concentration method has been known which is a so-called "pressurized floating concentration method". This is a solid/liquid separation method mainly used in a process of concentrating a sludge and utilizes a phenomenon that a sludge is floated when a pressure is exerted on the sludge to dissolve a gas, such as, air into the sludge and then liberated to adhere the gas of a foamed state to the sludge. However, this method has a problem of neccesitating a large amount of electric power for exerting a high pressure to the sludge and complicated equipment for practicing the method.
Also, an anaerobic digestion method has been known wherein a hot alkaline treating tank of accelerating the solubilization of an organic sludge by adding an alkaline to the sludge while heating the sludge is provided at a prestige of the anaerobic digestion tank which performs anaerobic digestion at a medium temperature of about 37.degree. C. during the anaerobic digestion treatment. This method has advantages that the digestion ratio is high and the amount of recovered methane is large. In this way, an organic sludge after the hot alkaline treatment and the anaerobic digestion treatment is subjected to the solid/liquid separation treatment at where it is separated to a concentrated sludge and a digested eluate, and the digested eluate is directly returned to a subsequent water treatment system. However, this method has a problem of containing a large amount of phosphorus and nitrogen in the digested eluate, so that the load of the water treatment system is largely increased.